A New Cluster Pair Method of Determining Absolute Single Ion Solvation Energies Demonstrated in Water and Applied to Ammonia

A new method of determining standard absolute solvation energies (enthalpies and Gibbs free energies) for individual ions is presented. This method originated from the cluster pair based approximation used in earlier work [Tissandier et al., J. Phys. Chem. A 1998, 102, 7787] and is called the cluster pair correlation scheme. Unlike the earlier approximation, the new scheme makes analysis possible in solvent systems for which bulk ion-solvation data are complemented by cluster ion data, either experimental or theoretical, for clusters containing only a single solvent molecule. The correlation scheme features linear plots of half the difference between standard conventional cluster solvation energies for anions and cations containing equal numbers of solvent molecules in the cluster vs their bulk counterparts. All of the correlation lines should intersect at a point whose ordinate defines the unknown shift between the conventional and absolute energy scales, the standard absolute energy of formation of the proton. The results obtained using this correlation scheme to analyze the available cluster ion data for water are in substantial agreement with the results reported earlier using the cluster pair based approximation. Preliminary values for the standard absolute enthalpy and Gibbs free energy of ammonation of the proton are obtained when the cluster pair correlation scheme is applied to ammonia, and a clear indication is obtained of what additional data are needed to improve these preliminary estimates.